The invention relates to plastic compositions having electrical conductivity. In particular, the invention relates to conductive thermoplastic compositions suitable for use in electrostatically painted articles.
It is known to impart electrical conductivity to plastic through the addition of a conductive filler, such as carbon black or carbon fibers, and thereby mold polymer articles that are particularly adapted for electrostatic painting. Electrostatic painting is an effective and desirable method of reducing manufacturing costs by reducing paint waste and polluting emissions, but it requires that the article to be painted be electrically conductive. Because plastic parts are generally insulating, the plastic article must be painted with a conductive primer or must be made conductive.
Painting nonconductive polymer parts with a conductive primer results in overspray, waste, and emissions of the primer itself and defeats many of the advantages of electrostatic painting. Use of a conductive primer may be avoided by adding a conductive filler such as conductive carbon black to the plastic composition. However, polymers tend to lose strength when even small amounts of carbon black are added. The prior art solutions have been to provide compositions that make the resulting plastic more ductile and flexible. For example, U.S. Pat. No. 5,484,838 to Helms et al. generally describes conductive blends of a crystalline polymer and a semi-crystalline or amorphous polymer. While such prior art compositions are sufficient for such applications as soft fascia, they are not suitable where higher strength and stiffness is needed, such as functional body panels, particularly for heavy duty vehicles such as trucks. What is needed is a polymer composition that has sufficient conductivity for electrostatic painting, yet is strong and stiff enough for heavy duty uses such as truck fenders, body panels, and the like.
A thermoplastic composition providing high strength and stiffness comprises: about 10 to about 50 weight percent polycarbonate; about 20 to about 60 weight percent polyester; about 0.005 to about 5 parts by weight transesterification quencher per 100 parts by weight polyester; about 1 to about 20 weight percent impact modifier; about 0.2 to about 20 weight percent conductive filler; and about 10 to about 40 weight percent glass fibers; wherein the composition after molding has a flexural modulus according to ASTM D790 not less than about 4xc3x97105 pounds per square inch (psi); and wherein all weight percents are based on the total weight of the composition.